The present invention relates to cryogenic refrigerators, in particular those operating to the Gifford McMahon or Solvay cycles or derivatives thereof, and more particularly to coldhead drive units thereof.
Coldheads of such cryogenic refrigerators may be broken down into two general category types, firstly those in which a displacer is mechanically actuated and secondly those in which a displacer is pneumatically actuated.
In the case of pneumatically actuated coldheads, these usually include a drive unit including valve means which has a rotary valve head cooperating with a static valve plate. At the interface between the valve head and valve plate, there are discrete ports which, by periodic alignment of the different ports, allow the passage of a working fluid to and from the regenerators and working volumes of the coldhead.
To maintain a necessary contact between the valve head and the valve plate, it is customary for the valve head surface distal to the valve plate to be subjected to a higher pressure than the effective pressure at the interface between the valve head and valve plate.
Furthermore, the diameter of the valve head must clearly be adequate to allow ports of sizes sufficient to allow for the passage of a flow of working fluid to and from the coldhead working volumes without significant restriction.
The valve head can be driven by a motor which must have sufficient torque to rotate the valve head which, for a given valve head to valve plate coefficient of friction, is dependent on the valve head/valve plate interface area, the interface diameter and the axial pressure differential operating on the valve head. Motor torque requirements are therefore very sensitive to the valve head diameter and axial pressure differential.
It has now been found that a modification to the valve means allows for an increase in the size of the valve head/plate interface to suit increasing sizes of coldhead without the need necessarily to increase the size of motor required to drive the valve head. This generally has the benefit of allowing a range of cryogenic refrigerators having different refrigerating capacities to employ a common valve drive motor. This has the advantage of reducing manufacturing costs and of allowing each member of the range to be connected to a common power supply. Furthermore, it generally reduces the valve head load against the valve plate, thereby reducing wear at the interface and increasing the life of these components.